1. Field of the Invention
The present invention relates to a connection sheet for firmly bonding an electronic component such as a semiconductor chip to a circuit board to achieve electrical connection between electrodes of the two components, and also to electrode connection structure and method using the connection sheet.
2. Description of the Related Art
With a recent trend to smaller-sized, thinner electronic components, electrical circuits used in such components have come to be increased in density and reduced in connection pitches. Since connection of an electronic component to fine electrodes is difficult to achieve with conventional techniques utilizing soldering, rubber connectors or the like, in recent years anisotropic conductive adhesives and film-like materials (connection sheets) which can provide excellent resolution are widely used.
A connection sheet comprises an adhesive having a predetermined content of an electrically conductive material, such as conducting particles. With the connection sheet interposed between an electronic component and electrodes or an electrical circuit, pressure or both heat and pressure are applied to the connection sheet, whereby the two components are firmly bonded together so that corresponding electrodes of the two components can be electrically connected to each other while providing insulation between adjacent electrodes.
The basic idea for achieving high resolution with the connection sheet is that the particle diameter of conducting particles in the vicinity of the electrodes shall be smaller than the length of an insulating region between adjacent electrodes, so as to ensure inter-electrode insulation performance. Also, the content of conducting particles in the connection sheet is selected to obtain a density such that the particles do not come into contact with one another, and that when electrodes are connected, the particles never fail to exist on the electrodes to be connected, to thereby achieve conductivity at connecting portions.
However, if the particle diameter of the conducting particles is too small, the conducting particles undergo cohesion and are united due to an excessive increase in the surface area of the particles, making it impossible to maintain required insulation performance between adjacent electrodes. On the other hand, if the content of the conducting particles is reduced, the number of conducting particles on the electrodes to be connected also decreases, with the result that conduction between corresponding electrodes cannot be achieved due to deficiency in the number of contact points. With conventional techniques, therefore, it is very difficult to achieve high resolution with a connection sheet while maintaining long-term connection reliability. Specifically, there has recently been an increasing demand for higher resolution, that is, for reduction in the area of electrodes and in the spacing between adjacent electrodes. When pressure or both heat and pressure are applied to the connection sheet in the connection step, however, the conducting particles on the electrodes flow together with the adhesive to a region between adjacent electrodes, thus impeding achievement of high resolution with the use of a connection sheet. If the viscosity of the adhesive is increased in order to suppress such outflow, then the conducting particles do not satisfactorily contact with each electrode, making it impossible to achieve electrical connection between electrodes facing each other. On the other hand, if the viscosity of the adhesive is decreased, not only the conducting particles are more likely to flow but also air bubbles are liable to be contained in the region between adjacent electrodes, lowering the connection reliability, especially, the moisture resistance.
In view of the drawbacks, Unexamined Japanese Patent Publications (KOKAI) No. 61-195179 and No. 4-366630, for example, disclose a multi-layer connection sheet including an insulating adhesive layer (first adhesive layer) and a layer (second adhesive layer) filled with conducting particles and separate from the first adhesive layer, wherein the viscosity of the second adhesive layer is selected such that the second adhesive layer exhibits relatively higher viscosity or cohesive force than the first adhesive layer at the time of connection, to lessen the flow of the conducting particles and thereby trap conducting particles on the electrodes.
According to the disclosed techniques, however, the layer filled with conducting particles has higher viscosity than the insulating adhesive layer at the time of connection; therefore, the conducting particles insufficiently contact with the electrodes, increasing the connection resistance and lowering the connection reliability. In order to reduce the connection resistance, a structure may be employed wherein the conducting particles are exposed in advance to the surfaces of the particle-filled layer so that they may be easily brought into contact with the electrodes. In this case, however, the particle diameter of the conducting particles must be increased, which makes it difficult to achieve high resolution.
There has also been proposed a connection sheet which permits connection to fine electrodes or a circuit and which provides excellent connection reliability, wherein conducting particles are concentrated at regions where electrodes are to be connected. Although this connection sheet permits connection to dot-like fine electrodes such as electrodes on a semiconductor chip, it is necessary that the particle-concentrated regions should be accurately aligned with corresponding dot-like electrodes, lowering the operation efficiency.